Electronic – Inequal discharge across battery cell

(This answer summarizes Anindo Ghosh's suggestions on the topic made in EE chat plus a few of my own observations. Please note that I am not considering this answer definitive: just want to add something else to the mix of suggestions)

This answer assumes (without justification) a LiPO battery-based design. Some background information:

• LiPO battery cells are 3.7V
• Connecting battery cells in parallel is undesirable because if one battery has lower internal resistance than the others, the first battery will discharge first and then the other batteries will begin charging the battery with the lowest internal resistance.

The recommendation suggestion is to split the circuit into the following parts:

• 5V circuit: for the ATmega and all the LED drivers. This part will be driven by a switching boost regulator to step up the voltage from 3.7V.
• Separated the LEDs into banks such that each bank is driven by set of serially-wired LiPO cells: this avoids having the LiPO cells wired in parallel (more on the reason for serial wiring below).

Since the TLC5940 is a sink driver, the LEDs can be wired at whatever voltage. Because these are RGB LEDs the \$V_{forward}\$ of at least some of the green and blue channels can be as much as 3.4, leaving only 0.3 V of headroom assuming given LiPO's nominal voltage. Adding the fact that LiPO voltage varies substantially over the discharge cycle (Wikipedia: "The voltage of a Li-poly cell varies from about 2.7 V (discharged) to about 4.23 V (fully charged)"), this is not sufficient. Furthermore, according to Anindo Ghosh and a TI thread, TLC5940 requires a good amount of headroom over the \$V_{forward}\$ voltage to properly regulate the LEDs: \$Vcc_{LED}\$ must be greater than \$Vforward_{LED}\$ by about 1.2 Volts at 120mA sinking current - see Figure 5 of the datasheet. This leads to the conclusion that each LED bank must be driven by two LiPO batteries which will yield a minimum 5.4 volts to drive the LEDs; after the \$V_{forward}\$ drop of 3.4 V there is still 2V of headroom left.

A possible alternative to consider is witing R, G, and B channels differently: since the R channel has lower \$Vforward_{LED}\$, it may be wired from a single LiPO cell, while G and B channels will still be powered by two serailly-connected cells.

Finally there was a suggestion to consider LiFePo4 batteries, which have tighter output voltage over the discharge cycle, offer longer cycle life and higher peak current.

Advice: 1) Your battery bank should be 48V (otherwise wire diameters will become unreasonable). Run through the calculations and you will see what I mean. For example, if you need, even momentarily, 50 Amps of 120V AC, that would be over 500 Amps at 12V.

2) Your charge controller must be designed for lithium batteries (or be programmable). Lithium batteries cannot be safely floated. You must stop charging them once charge is complete.

3) Investigate any building code issues prior to purchasing anything. If you are going to pull permits to do this, you will need approved plans. An inspector might not allow you to put in a large lithium ion battery pack without UL approvals or some such. If you live in the US you should be concerned about this. If you are outside the US, you are outside my experience.

4) When you put all those batteries together and charge or discharge them rapidly, the batteries in the middle will be unable to dissipate heat effectively. I don't know how big a problem this is but I would be very worried about it. I think you will need a thermal sensor near the middle of the pack to make sure the batteries do not overheat.

5) You cannot put the batteries in parallel without some type of over-current protection. It could be as simple as a fuse or PTC. The reason is, you don't want one bad cell (imagine it failing short circuit) to become a sink for all the other cells in parallel. This could make a bad situation much worse.

That is all I can think of at the moment. I can tell you right now that I would not do this. I would use AGM lead acid batteries. If you provide individual over voltage protection for each cell, that might help prevent the dangers of imbalance. When one cell in a series string stops accepting current, the whole series string stops. So the string would only be as good as the weakest cell.